new option: --mojitolib <path>

For @cardiy- this option specifies a mojito path other than <cwd>/node_modules/mojito

It works for apps in the mojito/examples/ dir, but I do not want to support this option for general use, because:

1. Only needed while old mojito installs an executable "mojito" in your $PATH
2. only save the effort of symlinking old mojito to <app>/node_modules/mojito
3. app devs should just `npm i`